1. Give specs and quality settings you're using
CPU: Phenom x4 9650 (2.3ghz)
RAM: 4Gb DDR2
GPU: ATI 6750 1GB DDR5
Played until Europolis in 1080p in High with AA in Low and everything else off.
After that i tried lowering both resolution and quality, No big change.
2. Say what kind of framerate range you mean when you say choppy
It peaked at 17fps in the lowest possible settings, no mather the config 4-9fps on highly detailed areas.
3. Give an exact location where you're experiencing this (careful of spoilers guys!)
4. And most importantly, try restart the game and see if the same thing still persists.
I noticed better performance when playing in my TV through HDMI. When i tried playing in my monitor through VGA it dropped fps. But restarting didn't help.